The present invention relates to a nuclear fuel assembly and, more particularly, to a nuclear fuel assembly for a boiling water reactor having spacer structure improved on heat transfer from fuel rods to the coolant.
Fuel assemblies for nuclear reactors have been improved on spacer structures thereof to increase heat transfer from the fuel rods to the coolant.
In a case of a pressurized water reactor (PWR) as disclosed in U.S. Pat. Nos. 3,395,077 and No. 3,379,619, spacer structures, used for holding the fuel rods to keep them laterally spaced from each other, are directed to improve the heat transfer. The structure is such that, in the center of four sides surrounding fuel rods, a grating type spacer and an obstacle serving as a vane are provided.
In this structure, the coolant flows over the peripheries of the fuel rods so as to cover them due to suitable configuration and mounting positions of the vanes. As a result, the coolant is agitated, the heat transfer increases, thereby raising allowable power level of a reactor core.
The above-described art relates to a PWR. If the spacer structure is used in a boiling water reactor, it is impossible to achieve the above-mentioned effect. In such cases, voids occur in the core of the BWR, and the coolant flows in a two-phase flow. Namely, a liquid film flow takes place on the surface of the fuel rod and a mixture of steam and liquid drops flow in a space region enclosed by fuel rods. Under this two-phase flow condition, if the above-mentioned conventional spacer is used, the coolant flows along the periphery of each fuel rod. Such a flow strips off the liquid film adhered to the fuel rod, thereby decreasing the amount of liquid adhered to the fuel rod in the form of liquid film. Namely, such a coolant flow is likely to cause boiling transition that nucleus boiling changes to film boiling. Therefore, power, that is, allowable power level at the boiling transition decreases.
An example of a BWR fuel assembly having a plurality of vanes is disclosed in U.S. Pat. No. 4,698,204.
The U.S. patent relates to a BWR fuel assembly having an intermediate flow mixing non-support grid. The grid does not support fuel rods to keep them spaced from each other and having vanes at all the fuel rod for mixing relatively cool coolant and relatively hot coolant.